Optical fiber transmission system with increased effective modal bandwidth transmission

ABSTRACT

A multi-mode optical fiber link is described. The multi-mode optical fiber link includes a first spatial mode converter that is coupled to a first single mode optical fiber. The first spatial mode converter conditions a modal profile of an optical signal propagating from the single mode optical fiber to the first spatial mode converter. A multi-mode optical fiber is coupled to the first spatial mode converter. A second spatial mode converter is coupled to an output of the multi-mode optical fiber and to a second single mode optical fiber. The second spatial mode converter reduces a number of optical modes in the optical signal. Both the first and the second spatial mode converters increase an effective modal bandwidth of the optical signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 10/605,107, filed on Sep. 9, 2003, entitled“Optical Transmitter for Increased Effective Modal BandwidthTransmission,” which claims priority to U.S. provisional patentapplication No. 60/481,166, filed on Aug. 1, 2003, entitled “OpticalFiber Transmission System with Increased Effective Modal Bandwidth.” Theentire disclosure of U.S. patent application Ser. No. 10/605,107 andU.S. provisional patent application No. 60/481,166 are incorporatedherein by reference.

BACKGROUND OF INVENTION

Many existing optical fiber transmission systems use multi-mode opticalfiber. Multi-mode optical fiber is widely used because it is relativelyinexpensive, easy to install and because it is suitable for use with lowcost transmitter and receiver components. The relatively large opticalfiber core and numerical aperture of multi-mode optical fibers allowsmore light to be launched into the optical fiber, as compared tosingle-mode optical fibers. Therefore, such systems can use lower powerand lower cost optical sources. For these reasons, local area networkshave employed multi-mode optical fiber for many years. Some datacommunication systems, such as Fiber Data Distribution Interface (FDDI)systems are specifically designed to use multi-mode optical fiber. Knownmulti-mode optical fiber transmission systems, however, have relativelylow bandwidth-distance products for a given bit error rate (BER) and,therefore, are not suitable for many state-of-the art communicationsystems.

BRIEF DESCRIPTION OF DRAWINGS

This invention is described with particularity in the detaileddescription. The above and further advantages of this invention may bebetter understood by referring to the following description inconjunction with the accompanying drawings, in which like numeralsindicate like structural elements and features in various figures. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

FIG. 1 illustrates a block diagram of a multi-mode optical fibertransmission system that includes two spatial mode optical convertersaccording to the present invention.

FIG. 2 illustrates a block diagram of a single-mode optical fibertransmission system that includes a spatial mode optical converteraccording to the present invention.

FIG. 3A is a schematic representation of the first spatial modeconverter that couples the single mode optical fiber and the multi-modeoptical fiber according to the present invention.

FIG. 3B illustrates an electric field diagram of an optical signalpassing from the single mode optical fiber to the multi-mode opticalfiber according to the present invention.

FIG. 4 illustrates a block diagram of an optical transmitter thatincludes an electro-absorption modulator according to the presentinvention that generates optical signals with improved or optimalspectral and phase characteristics for transmission through an opticalfiber link.

FIG. 5 illustrates a block diagram of an optical transmitter thatincludes an electro-absorption modulated laser (EML) according to thepresent invention that generates optical signals with improved oroptimal spectral and phase characteristics for transmission through anoptical fiber link.

FIG. 6 illustrates a block diagram of an optical transmitter thatincludes an embodiment of a laser modulator according to the presentinvention that generates optical signals with improved or optimalspectral and phase characteristics for transmission through an opticalfiber link.

FIG. 7 illustrates a block diagram of one embodiment of an opticalreceiver for the optical fiber transmission system with increasedeffective modal bandwidth according to the present invention thatincludes dynamic re-optimization and electronic dispersion compensation.

DETAILED DESCRIPTION

The present invention relates to methods and apparatus for increasingthe effective modal bandwidth of optical fiber transmission systems. Theterm “effective modal bandwidth” is defined herein to mean thebandwidth-distance product of the transmission system for a given BitError Rate (BER) and/or a certain transmission specification. Increasingthe effective modal bandwidth of a multi-mode optical fiber transmissionsystem will allow providers to increase the data rate and will extendthe useful service life of many installed multi-mode optical fibertransmission systems.

One aspect of the present invention is embodied in the design of opticaltransmitters that have improved or optimum spectral and phasecharacteristics for transmitting data in a multi-mode optical fiber.Another aspect of the present invention is embodied in the use ofspatial filtering to reduce the number of modes propagating in amulti-mode optical fiber. These aspects alone or in combination increasethe effective modal bandwidth of multi-mode optical fiber transmissionsystems.

FIG. 1 illustrates a block diagram of a multi-mode optical fibertransmission system 100 that includes two spatial mode opticalconverters according to the present invention. The transmission system100 includes an optical transmitter 102, a multi-mode optical fiber link104, and an optical receiver 106. The optical transmitter 102 generatesoptical signals for data transmission through the multi-mode opticalfiber link 104.

In one embodiment, the optical transmitter 102 includes an intensitymodulated optical source, an electro-absorption modulated laser, anintegrated laser modulator, or a laser modulator having parameters thatgenerate optical signals with improved or optimal spectral and phasecharacteristics for transmission through an optical fiber link asdescribed herein. In one embodiment, the optical transmitter 102includes more than one optical source that generates additional opticalsignals at different wavelengths. In one embodiment, the optical sourceincludes a WDM optical source that generates a plurality of opticalsignals and each of the plurality of optical signals has a differentwavelength.

In some embodiments, the optical transmitter 102 includes additionaloptical sources that are used to generate additional optical signalsthat increase the data capacity of the multi-mode optical fiber link104. In some of these embodiments, the multi-mode optical fibertransmission system 100 includes additional optical transmitters 102that are used to generate optical signals that propagate in oppositedirections in the same multi-mode optical fiber. Separate opticalcarriers can be used to minimize crosstalk between optical signalspropagating in opposite directions.

The optical transmitter 102 is optically coupled to a first single-modeoptical fiber 108. Optical signals generated by the optical transmitter102 propagate down the first single-mode optical fiber 108. A firstspatial mode converter 110 is optically coupled to the first single-modeoptical fiber 108. The first spatial mode converter 110 conditions themodal profile of the optical signal propagating through the firstspatial mode converter 110. The first spatial mode converter 110 cancondition the modal profile in many ways in order to increase theeffective bandwidth of the optical signal or to increase otherperformance metrics of the transmission system 100. For example, thefirst spatial mode converter 110 can condition the modal profile of theoptical signal to reduce phase and sideband information, noise, or modaldispersion in the optical signal.

An input 112 of the multi-mode optical fiber link 104 is opticallycoupled to the first spatial mode converter 110. The multi-mode opticalfiber link 104 can include a single length of multi-mode optical fiberor can include multiple lengths of multi-mode optical fiber that arecoupled together. The multiple lengths of multi-mode optical fiber canbe butt coupled together. For example, the butt couplings can be taperedoptical fiber sections or polished optical fiber sections.

In one embodiment, the multi-mode optical fiber link 104 includes atleast one single mode optical fiber link section. The single modeoptical fiber link section can be a long haul optical fiber link thatconnects distant networks. In this embodiment, the transmission system100 of the present invention can be used to link multiple enterprisenetworks that are separated by long distances.

A second spatial mode converter 116 is optically coupled to an output114 of the multi-mode optical fiber link 104. The second spatial modeconverter 116 is also optically coupled to a second single-mode opticalfiber 118. The second spatial mode converter 116 reduces the number ofmodes in the optical signal that are transmitted through the secondspatial mode converter 116 and, therefore, limits the number of dominantmodes that are received by the optical receiver 106.

The second spatial mode converter 116 can reduce the number ofhigher-order modes, the number of lower-order modes or both the numberof higher-and lower-order modes in the optical signal propagatingthrough the second spatial mode converter 116. By lower-order modes, wemean modes in which most of the energy is localized around the center ofthe optical fiber core of the multi-mode optical fiber. By higher-ordermodes, we mean modes in which most of the energy is localized outside ofthe center of the optical fiber core of the multi-mode optical fiber.

Both the first 110 and the second spatial mode converters 116 increasethe effective modal bandwidth of the multi-mode optical fibertransmission system 100. The first spatial mode converter 110 can be anytype of spatial mode converter that conditions the modal profile. Thesecond spatial mode converter 116 can be any type of spatial modeconverter that reduces the number of modes in the optical signalgenerated by the optical transmitter 102. For example, the first andsecond spatial mode converters 110, 116 can include a fusion splice or abutt coupling between the multi-mode optical fiber link 104 and arespective one of the first 108 and the second single-mode optical fiber118. The butt coupling can be positioned at a bulkhead. The first andsecond spatial mode converters 110, 116 can also include a lens imagingsystem having refractive and diffractive elements.

The effective modal bandwidth of the multi-mode optical fibertransmission system 100 according to the present invention including thetwo spatial mode converters 110, 116 has a relatively high-level ofimmunity to polarization effects, fiber stress, vibration, and changesin temperature. In particular, there is little or no change in theeffective modal bandwidth due to changes in laser polarization orchanges in polarization caused by mechanical stress on the multi-modeoptical fiber link 104. Also, there is little or no change in theeffective modal bandwidth due to temperature changes in the fiberenvironment.

In one embodiment, the receiver 106 includes electronic dispersioncompensation. In this embodiment, the receiver 106 includes at least oneactive electrical filter that is electrically coupled to the output of adetector as described herein. Also, in one embodiment, the receiverincludes dynamic re-optimization that automatically adjusts at least onereceiver parameter in order to compensate for changes in an averagepower of the received optical signal as described herein.

The present invention features a method of increasing effective modalbandwidth of an optical signal transmitted through a multi-mode opticalfiber. The method includes generating an optical signal and propagatingthe optical signal through a single-mode optical fiber. In oneembodiment, the optical signal is chosen to reduce phase corruption. Theoptical signal is then spatially mode converted to an optical signalhaving a conditioned modal profile. The spatial mode converting reducesmodal dispersion, which increases an effective bandwidth of the opticalsignal.

The optical signal having the lower number of modes is then propagatedthrough a multi-mode optical fiber. The optical signal propagatingthrough the multi-mode optical fiber is then spatially mode converted,which further increases the effective bandwidth of the optical signal.The spatial mode conversions can reduce changes in effective modalbandwidth of the optical signal that are caused by physical effects,such as thermal variations in the multi-mode optical fiber, polarizationeffects in the multi-mode optical fiber, mechanical stress in themulti-mode optical fiber, optical fiber splices in the multi-modeoptical fiber, and optical connector misalignment in the multi-modeoptical fiber.

Some aspects of the present invention are described in connection with amulti-mode optical fiber link that is typically a local area fiber link.However, the present invention can also be practiced with a single-modeoptical fiber link that is typically a long-haul optical fiber link.

FIG. 2 illustrates a block diagram of a single-mode optical fibertransmission system 150 that includes a spatial mode optical filteraccording to the present invention. The transmission system 150 includesan optical transmitter 102, a single-mode optical fiber link 152, and anoptical receiver 106. The single-mode optical fiber transmission system150 is similar to the multi-mode optical fiber transmission system 100that was described in connection with FIG. 1.

The optical transmitter 102 generates optical signals for datatransmission through the single-mode optical fiber link 152. In oneembodiment, the optical transmitter 102 includes more than one opticalsource that generates additional optical signals at differentwavelengths that increase the data capacity of the single-mode opticalfiber link 152. In some embodiments, the single-mode optical fibertransmission system 100 includes additional optical transmitters 102that are used to generate optical signals that propagate in oppositedirections in the same single-mode optical fiber.

The optical transmitter 102 is optically coupled to a first single-modeoptical fiber 108. Optical signals generated by the optical transmitter102 propagate down the first single-mode optical fiber 108. A firstspatial mode converter 110 is optically coupled to the first single-modeoptical fiber 108. The first spatial mode converter 110 conditions themodal profile in the optical signal propagating through the firstspatial mode converter 110.

An input 151 of the single-mode optical fiber link 152 is opticallycoupled to the first spatial mode converter 110. The single-mode opticalfiber link 152 can include a single length of single-mode optical fiberor can include multiple lengths of single-mode optical fiber that arefusion spliced or coupled together. An optical coupler 154 is opticallycoupled to an output 156 of the single-mode optical fiber link 152. Theoptical coupler 154 is also optically coupled to a second single-modeoptical fiber 118.

The first spatial mode converter 110 increases the effective modalbandwidth of the single-mode optical fiber transmission system 150. Theeffective modal bandwidth of the single-mode optical fiber transmissionsystem 150 according to the present invention including the firstspatial mode converter 110 has a relatively high-level of immunity topolarization effects, fiber stress, and changes in temperature.

In one embodiment, the receiver 106 includes an electronic dispersioncompensation circuit. In this embodiment, the receiver 106 includes atleast one active electrical filter that is electrically coupled to theoutput of a detector as described herein. Also, in one embodiment, thereceiver includes dynamic re-optimization that automatically adjusts atleast one receiver parameter in order to compensate for changes in anaverage power of the received optical signal as described herein.

FIG. 3A is a schematic representation 170 of the first spatial modeoptical converter 110 that couples the single mode optical fiber 108 andthe multi-mode optical fiber link 104 according to the presentinvention. The single mode optical fiber 108 is designed to propagate anoptical signal having a wavelength with only one type of spatialdistribution (i.e. one optical mode). Single-mode optical fiberstypically have a core 172 that is between about 8-10 microns indiameter.

The single mode optical fiber 108 is coupled to an input 174 of thefirst spatial mode converter 110. The first spatial mode converter 110can be any type of spatial mode converter that conditions the modalprofile of the optical signal that is applied to the input 174. Forexample, the first spatial mode converter 110 can include a fusionsplice, a butt coupling, or a lens imaging system having refractive anddiffractive elements. An output 176 of the spatial mode converter 110 iscoupled to the multi-mode optical fiber link 104. The multi-mode opticalfiber link 104 is designed to support multiple spatial distributions.Multi-mode optical fibers typically have a core 178 that is on the orderof 50 microns in diameter.

In one embodiment, the first spatial mode converter 110 couples thesingle mode optical fiber 108 and the multi-mode optical fiber link 104so as to achieve a predetermined offset between a center of the core 172of the first single mode optical fiber 108 and a center of a core 178 ofthe multi-mode optical fiber in the link 104. For example, in thisembodiment, the center of the core 172 of the first single mode opticalfiber 108 can be offset between about 15-25 micrometers from the centerof the core 178 of the multi-mode optical fiber in the link 104.Offsetting the center of the core 172 of the first single mode opticalfiber 108 from the center of the core 178 of the multi-mode opticalfiber in the link 104 changes the launch conditions.

FIG. 3B illustrates an electric field diagram 180 of an optical signalpassing from the single mode optical fiber 108 to the multi-mode opticalfiber link 104 according to the present invention. The electric fielddiagram 180 illustrates the magnitude of the electric field in theoptical signal as a function of distance in microns from inside thesingle mode optical fiber 108 to inside the multi-mode optical fiberlink 104.

The magnitude of the electric field intensity 182 in the optical signalat Z=0 micrometers corresponds to the magnitude of the electric fieldintensity inside the single mode optical fiber 108 and at the input 174of the spatial mode converter 110. The magnitude of the electric fieldintensity 184 in the optical signal at Z=1,000 micrometers correspondsto the magnitude of the electric field intensity at the mode conversionpoint inside the spatial mode converter 110. The magnitude of theelectric field intensity 186 in the optical signal at Z=2,000micrometers corresponds to the magnitude of the electric field intensityat the output 174 of the spatial mode converter 110. The magnitude ofthe electric field intensity 188 in the optical signal at Z=4,000micrometers corresponds to the magnitude of the electric field intensityinside of the multi-mode optical fiber link 104.

The electric field diagram of an optical signal passing from themulti-mode optical fiber link 104 to the second single-mode opticalfiber 118 is similar to the electric field diagram 180 of FIG. 3B, butthe distance scale is inverted. The spatial mode converters 110, 116condition and reduce the number of dominant modes that are received bythe optical receiver 106.

FIG. 4 illustrates a block diagram of an optical transmitter 200 thatincludes an electro-absorption modulator according to the presentinvention that generates optical signals with improved or optimalspectral and phase characteristics for transmission through an opticalfiber link. The optical transmitter 200 improves the spectral and phasecharacteristics for transmission through multi-mode optical fiber links,such as the multi-mode optical fiber link 104 that is described inconnection with FIG. 1. In addition, the optical transmitter 200improves the spectral and phase characteristics for transmission throughsingle-mode optical fiber links, such as long-haul single-mode opticalfiber links.

The optical transmitter 200 is designed to generate optical signals thathave specific characteristics which increase or maximize immunity tovariations in the phase of the optical signal received by the opticalreceiver 106 (FIGS. 1 and 2). One characteristic of the optical signalgenerated by the optical transmitter 200 is a reduction in time varyingphase or sideband information in the transmission spectrum of theoptical signal. Another characteristic of the optical signal generatedby the optical transmitter 200 is a reduction in the phase informationthat is required to transmit the data in the optical fiber links 104,152 (FIGS. 1 and 2).

Another characteristic of the optical signal generated by the opticaltransmitter 200 is a reduction or elimination of mixing that is requiredat the optical receiver 106 (FIG. 1) to recover the optical signal. Yetanother characteristic of the optical signal generated by the opticaltransmitter 200 is an increase in isolation of optical signals reflectedback towards the optical transmitter 102. In one embodiment of theinvention, the optical transmitter 200 generates an optical signal withone or any combination of these characteristics. Generating an opticalsignal with one or more of these characteristics will increase theeffective modal bandwidth of the multi-mode optical fiber transmissionsystem 100 (FIG. 1) and the effective modal bandwidth of the single-modeoptical fiber transmission system 150 (FIG. 2).

One type of optical transmitter that can generate an optical signal withone or any combination of these characteristics is anelectro-absorptively (EA) modulated optical transmitter. The opticaltransmitter 200 illustrated in FIG. 4 is an exemplary EA opticalmodulated transmitter. Numerous types of EA modulated sources can beused in an optical transmitter according to the present invention. Inother embodiments, other types of intensity modulators are used.

The optical transmitter 200 includes a laser 202 that generates acontinuous wave (CW) optical signal at an output 204. In someembodiments, the laser 202 is a semiconductor diode laser. However,other types of lasers can also be used. The transmitter 200 alsoincludes a laser bias circuit 206. An output 208 of the laser biascircuit 206 is electrically connected to a bias input 210 of the laser202. The laser bias circuit 206 generates a current at the output 208that biases the laser 202.

The optical transmitter 200 also includes an Electro-AbsorptionModulator (EAM) 210 that modulates the CW optical signal generated bythe laser 202. In some embodiments, the laser 202 and the EAM 210 areseparate discrete components. In other embodiments, the laser 202 andthe EAM 210 are physically integrated on a single substrate. The EAM 210includes an optical input 212, a bias and modulation input 214, and anoptical output 216. The optical input 212 is positioned in opticalcommunication with the output 204 of the laser 202. A waveguide, such asan optical fiber, can be used to optically couple the output 204 of thelaser 202 to the optical input 212 of the EAM 210.

The optical transmitter 200 including the EAM 210 generates opticalsignals with improved or optimal spectral and phase characteristics fortransmission through a multi-mode optical fiber link. The modulatedoptical signal that is generated by the optical transmitter 200including the EAM 210 has very little phase information because EAmodulators operate as efficient intensity modulators.

In one embodiment of the invention, the EAM 210 is specifically designedand fabricated to have at least one parameter that causes the EAM 210 tomodulate intensity so as to suppress time varying phase and sidebandinformation in the transmission spectrum. EA modulators are relativelyefficient intensity modulators. Therefore, time varying phase andsideband information in the transmission spectrum is generallysuppressed. However, a transmitter according to one embodiment of theinvention can be designed, fabricated, and/or operated to further reducetime varying phase and sideband information in the transmissionspectrum.

There are numerous physical EA modulator parameters that can be adjustedto change the amplitude and phase characteristics of the modulatedoptical signal in order to suppress phase and sideband information fromthe transmission spectrum. For example, parameters, such as theextinction ratio or voltage swing of the EA modulator, polarizationproperties, the 3-dB bandwidth, the facet coating properties, the inputthird-order intercept (IIP3), and the spurious free dynamic range (SFDR)can be adjusted during design and fabrication to suppress phase andsideband information from the transmission spectrum. Adjusting theextinction ratio of the EA modulator has been shown to suppress phaseand sideband information from the transmission spectrum and,consequently, to increase the signal-to-noise ratio of optical signalspropagating through multi-mode optical fiber. In one embodiment of theinvention, the extinction ratio of the EA modulator 210 is in the rangeof about five to fifteen.

The optimal value of the extinction ratio is a function of the length ofthe multi-mode optical fiber. The optimal value of the extinction ratiocan also be a function of the number of the fiber connectors and thealignment of the fiber connectors in the multi-mode optical fiber link104 (FIG. 1) and the single mode optical fiber link 152 (FIG. 2). Inaddition, the optimal value of the extinction ratio can also be afunction and many environmental factors, such as the level of thevibration, mechanical strain, thermal shock, and optical powerfluctuations in the optical fiber link.

In one example, an EA modulator with an extinction ratio of about 11.5has been shown to transmit optical signals through a 1250 footmulti-mode optical fiber link with relatively low phase and sidebandinformation and relatively high signal-to-noise ratio compared with EAmodulators having extinction ratios of about five and about eight in thesame optical fiber link under similar environmental conditions. Inanother example, an EA modulator with an extinction ratio of about tenhas been shown to transmit optical signals through a 4500 footmulti-mode optical fiber link with relatively low phase and sidebandinformation and relatively high signal-to-noise ratio compared with anEA modulator having an extinction ratio of about five in the sameoptical fiber link under similar environmental conditions.

The optical transmitter 200 also includes a bias and data multiplexingcircuit 218 that generates the desired electrical bias and data signalsfor the EAM 210. In some embodiments, the bias and data multiplexingcircuit 218 includes two physically separate components. In otherembodiments, the bias and data multiplexing circuit 218 is one componentas shown in FIG. 4. An output 220 of the bias and data multiplexingcircuit 218 is electrically connected to the modulation input 214 of theEAM 210. The EAM 210 modulates the CW optical signal generated by thelaser 202 with an electronic data signal generated by the bias and datamultiplexing circuit 218. The modulated optical signal propagates fromthe optical output 216 of the EAM 210.

In one embodiment of the invention, the operating conditions of the EAM210 are chosen so as to suppress phase and/or sideband information inthe transmission spectrum generated by the EAM 210. For example, theoperating temperature of the EAM 210 and the bias voltage that isgenerated by the bias and data multiplexing circuit 218 and applied tothe modulation input 214 of the EAM 210 can be adjusted during operationto suppress phase and/or sideband information from the optical signal.

In addition, parameters of the laser 202 that generates the opticalsignal which is modulated by the EAM 210 can be adjusted to suppressphase and/or sideband information from the transmission spectrum. Forexample, parameters, such as the wavelength and the optical modestructure of the optical signal generated by the laser 202 can beadjusted so as to suppress phase information and/or sideband informationfrom the modulated optical signal.

The modulated optical signal that is generated by the opticaltransmitter 200 including the EAM 210 has certain characteristics in itstransmission spectrum that increase the effective modal bandwidth of theoptical fiber link. For example, one characteristic of the transmissionspectrum is that the optical signal has minimal time varying phase.Another characteristic of the transmission spectrum is that it hasminimal sideband information.

The modulated optical signal that is generated by the opticaltransmitter 200 requires essentially no phase information to transmitthe data in an optical link, such as the multi-mode optical fiber link104 (FIG. 1) and the single-mode optical fiber link 152 (FIG. 2). Inaddition, the modulated optical signal that is generated by the opticaltransmitter 200 has good isolation from optical signals reflecting backtowards the optical transmitter 200.

Thus, the optical transmitter 200 improves the spectral and phasecharacteristics for transmission through multi-mode optical fiber links,such as the multi-mode optical fiber link 104 that is described inconnection with FIG. 1. In addition, the optical transmitter 200improves the spectral and phase characteristics for transmission throughsingle-mode optical fiber links, such as the single-mode optical fiberlink 152 that is described in connection with FIG. 2.

There are numerous other types of optical transmitters that whendesigned, fabricated, and operated according to the present inventionwill generate optical signals with improved or optimal spectral andphase characteristics for transmission through a multi-mode andsingle-mode optical fiber link. These optical transmitters includeelectro-absorption modulated lasers (EMLs), laser modulators, andelectro-optic modulators.

FIG. 5 illustrates a block diagram of an optical transmitter 250 thatincludes an electro-absorption modulated laser (EML) 252 according tothe present invention that generates optical signals with improved oroptimal spectral and phase characteristics for transmission through anoptical fiber link. The EML 252 includes a laser diode 254 section andan electro-absorption modulator (EAM) 256 section.

The laser diode 254 section is typically a distributed feedback (DFB)laser. The EAM 256 is typically a device that includes a semiconductorlayer, such as a multi-quantum well semiconductor layer. Thesemiconductor layer typically has a slightly larger absorption band edgethan the photon energy of the light being modulated. The laser diode 254section is optically coupled to the EAM 256 section. The laser diode 254section and EAM 256 section are typically integrated onto a singlesubstrate, but can be physically separate devices.

A laser bias circuit 258 has an output 260 that is electrically coupledto a bias input 262 of the laser diode 254. The laser bias circuit 258generates a continuous wave (CW) current that drives the laser diode254, thereby causing the laser diode 254 to emit substantiallymonochromatic light of a predetermined wavelength.

A modulator bias and data multiplexing circuit 264 has an output 266that is electrically coupled to a modulation input 268 of the EAM 256.The modulator bias and data multiplexing circuit 264 generates a voltageacross the multi-quantum well semiconductor layer that produces areverse bias modulating electric field across the semiconductor layer ofthe EAM 256. The reverse bias modulating electric field causes theabsorption edge of the semiconductor layer of the EAM 256 to reversiblymove to a longer wavelength, which corresponds to a lower absorptionedge. The lower absorption edge causes the semiconductor layer of theEAM 256 to absorb the light generated by the laser diode 254 sectionthat propagates through the semiconductor layer of the EAM 256.

Reducing the voltage across the multi-quantum well semiconductor layerresults in the elimination or reduction of the reverse bias electricfield, which causes the semiconductor layer of the EAM 256 to allowlight generated by the laser diode 254 to transmit through thesemiconductor layer of the EAM 256. Therefore, light emitted from thelaser diode 254 that propagates to the EAM 256 is modulated bymodulating the voltage across the multi-quantum well semiconductor layerof the EAM 256. The light emitted is modulated between a sufficientreverse bias voltage across the semiconductor layer that causes thelayer to be substantially opaque to the light emitted from the laserdiode 254, and substantially zero or a sufficiently positive biasvoltage that causes the layer to be substantially transparent to thelight emitted from the laser diode 254.

The resulting modulated light is emitted at an optical output 270 of theEML 252. The optical output 270 is directly coupled to the single-modeoptical fiber 108 (FIGS. 1 and 2). The wavelength of the modulated lightcan be controlled by adjusting the amplitude of the CW current generatedby the laser bias circuit 258 and applied to the laser diode 254. Thewavelength of the modulated light can also be controlled by adjustingthe temperature of the laser diode 254.

The EML 252 includes a thermoelectric cooler (TEC) 272 that controls thetemperature of the laser diode 254 and the EAM 256. The temperature ofthe EML 252 can be stabilized by using a thermal sensor 274 and afeedback circuit 276. The thermal sensor 274 is thermally coupled to thelaser diode 254 and is electrically coupled to the feedback circuit 276.The feedback circuit 276 is electrically coupled to the TEC 272. Thefeedback circuit 276 receives a signal from the thermal sensor 274 thatis related to the temperature of the laser diode 254 and generates asignal in response to the temperature. The signal generated by thefeedback circuit 276 controls the thermal properties of the TEC 272 tomaintain the laser diode 254 at a predetermined operating temperature(and thus the major portion of spectral energy of the emitted light atthe desired wavelength) independent of ambient temperature.

In one embodiment of the invention, the EML 252 is specifically designedand fabricated to have at least one parameter that causes the EML 252 togenerate a transmission spectrum with suppressed phase and sidebandinformation. There are numerous physical EML parameters that can beadjusted to change the amplitude and phase characteristics of themodulated optical signal in order to suppress phase and sidebandinformation from the transmission spectrum.

For example, parameters of the EAM 256, such as the extinction ratio,the polarization properties, the 3-dB bandwidth, the modulator chirp,the optical mode structure, the input third-order intercept (IIP3), thespurious free dynamic range (SFDR), and the output facet coatingproperties can be adjusted during design and fabrication to suppressphase and/or sideband information from the transmission spectrum.

Also, parameters of the laser diode 254, such as the wavelength, theoptical mode structure, and the parameters of the output facet coatingcan be adjusted during design and fabrication to suppress phase and/orsideband information from the transmission spectrum. In addition,parameters specific to EML devices, such as the electrical isolation andthe optical coupling between the laser diode 254 and the EAM 256 can beadjusted during design and fabrication to suppress phase and/or sidebandinformation from the transmission spectrum.

In one embodiment of the invention, the operating conditions of the EML252 are chosen so as to suppress phase and/or sideband information inthe transmission spectrum. For example, operating conditions, such asthe current generated by the laser bias circuit 258 and the resultingoptical power received by the EAM 256, the bias voltage swing that isgenerated by the bias and data multiplexing circuit 264 and received bythe EAM 256, and the operating temperature of the laser diode 254 andthe EAM 256 can be adjusted during operation of the EML 252 to suppressphase and/or sideband information in the transmission spectrum.

The present invention can also be practiced with numerous types of lasermodulators. FIG. 6 illustrates a block diagram of an optical transmitter300 that includes an embodiment of a laser modulator 302 according tothe present invention that generates optical signals with improved oroptimal spectral and phase characteristics for transmission through anoptical fiber link. The optical transmitter 300 includes a laser section304 and a modulator section 306.

The laser section 304 of the laser modulator 302 shown in FIG. 6 is atunable three section Distributed Bragg Reflector (DBR) laser. In otherembodiments (not shown), a single section DFB laser can be used ifwavelength tuning is not desirable. The laser section 304 includes again section 308, a phase section 310, and a grating section 312 thatare butted together. The gain section 308 generates an optical signal.The phase section 310 introduces an optical phase shift to tune thelaser wavelength. The grating section 312 forms a DBR mirror.

A high reflection coating 314 is deposited on one side of the gainsection 308. A laser cavity is formed between the high reflectioncoating 314 and the DBR mirror formed by the grating section 312. Theoptical transmitter 300 includes a laser bias circuit 316 having anoutput 318 that is electrically connected to a bias input 320 of thegain section 308. The laser bias circuit 316 generates a current at theoutput 318 that biases the gain section 308 to emit the desired opticalsignal. The design and operation of such lasers are well known in theart.

The modulator section 306 of the laser modulator 302 is positionedoutside of the laser cavity beyond the DBR mirror in the grating section312. Forming the modulator section 306 external to the laser cavityintroduces relatively low wavelength chirp into the modulated opticalsignal. An input 328 of the modulator section 306 is optically coupledto the grating section 312. An output facet 330 of the modulator section306 transmits the modulated optical signal. An anti-reflection coating332 is deposited on the output facet 330 of the modulator section 306 toprevent undesired reflection from entering the laser cavity.

A modulator bias and data multiplexing circuit 322 has an output 324that is electrically coupled to a modulation input 326 of the modulatorsection 306. The modulator section 306 is an intensity modulator thatmodulates a CW optical signal that is generated by the laser section 304with the data generated by the modulator bias and data multiplexingcircuit 322. The modulated optical signal is transmitted though theoutput facet 330 of the modulator section 306 and the anti-reflectioncoating 332.

Many different types of modulator sections 306 can be used with anoptical transmitter 300. For example, the modulator section 306 can be aFranz-Keldysh-type electro-absorption modulator section. Such amodulator section 306 includes a section of waveguide with an activeregion of bulk semiconductor heterostructure material having a slightlylarger bandgap energy than the photon energy of the optical signal beingmodulated. When the modulator bias and data multiplexing circuit 322applies a reverse bias field to the modulation input 326 of themodulator section 306, the absorption edge is lowered, thus reducing thelight emitted.

The modulator section 306 can also be a modulated amplifier-typemodulator. Such a modulator includes a gain section that can be formedof the same material as the gain section 308 in the laser cavity.Modulated amplifier-type modulators can achieve relatively broad opticalbandwidth. In addition, the modulator section 306 can be aguide/antiguide-type modulator. Guide/antiguide modulators userefractive index effects to achieve intensity modulation. However,unlike other devices that use refractive index effects, such asMach-Zehnder type modulators, these modulators do not generate largeamounts of phase and sideband information in the transmission spectrumbecause they do not use interference effects.

In one embodiment of the invention, the optical transmitter 300 isspecifically designed and fabricated to have at least one parameter thatcauses the optical transmitter 300 to generate a transmission spectrumwith suppressed phase and sideband information. There are numerousphysical parameters of the laser section 304 and the modulator section306 that can be adjusted to change the amplitude and phasecharacteristics of the modulated optical signal in order to suppressphase and sideband information from the transmission spectrum.

For example, parameters of the laser section 304, such as thewavelength, the optical mode structure, and the parameters of the outputfacet coating can be adjusted during design and fabrication to suppressphase and/or sideband information from the transmission spectrum. Inaddition, parameters specific to DBR and DFB laser devices, such as thegrating parameters and the properties of the waveguides in the gainsection 308, the phase section 310, and the grating section 312, as wellas the coupling parameters between these sections, can be adjustedduring design and fabrication to suppress phase and/or sidebandinformation from the transmission spectrum.

Also, parameters of the modulator section 306, such as the extinctionratio, the polarization properties, the 3-dB bandwidth, the modulatorchirp, the optical mode structure, the input third-order intercept(IIP3), the spurious free dynamic range (SFDR), the lateral index guideand antiguide profiles (for guide/antiguide-type modulators), and theoutput facet coating properties can be adjusted during design andfabrication to suppress phase and/or sideband information from thetransmission spectrum.

In one embodiment of the invention, the operating conditions of theoptical transmitter 300 are chosen so as to suppress phase and/orsideband information in the transmission spectrum. For example, thecurrent generated by the laser bias circuit 316 and the resultingoptical power received by the modulator section 306, the bias voltagethat is generated by the bias and data multiplexing circuit 322 andreceived by the modulator section 306, and the operating temperature ofthe laser section 304 and the modulator section 306 can be adjustedduring operation to suppress phase and/or sideband information from thetransmission spectrum.

The optical transmitters described herein that generate optical signalswith improved or optimal spectral and phase characteristics fortransmission through an optical fiber link can be used for transmitting10GEthernet data in multi-mode optical fiber transmission systemsgreater than 300 meters long. Error free transmission of optical signalshaving a 1310 nm wavelength over 300 meters of multi-mode optical fiberusing such optical transmitters has been demonstrated.

FIG. 7 illustrates a block diagram of one embodiment of an opticalreceiver 350 for the optical fiber transmission system with increasedeffective modal bandwidth according to the present invention thatincludes dynamic re-optimization and electronic dispersion compensation.The dynamic re-optimization and electronic dispersion compensation aloneor in combination increase the bandwidth-distance product of the opticalfiber transmission system according to the present invention.

The optical receiver 350 includes a photo-detection circuit 352 that isoptically coupled to the output of the second single-mode optical fiber118 (FIGS. 1 and 2). The photo-detection circuit 352 includes aphoto-diode that converts the received optical signal into an electricalsignal. The optical receiver 350 also includes a pre-amplifier 354having an input 356 that is electrically connected to a signal output358 of the photo-detection circuit 352. The pre-amplifier 354 amplifiesthe electrical signal generated by the photo-detection circuit 352 to asignal level that is suitable for electronic processing.

The optical receiver 350 also includes a voltage sensing amplifier 360having an input 362 that is electrically connected to a control output364 of the photo-detection circuit 352. The voltage sensing amplifier360 generates a feed forward signal having a voltage that isproportional to the average optical power level of the received opticalsignal. A decision threshold circuit 366 has a signal input 368 that iselectrically connected to an output 370 of the pre-amplifier 354 and acontrol input 372 that is electrically connected to an output 374 of thevoltage sensing amplifier 360. The decision threshold circuit 366adjusts the decision threshold in the optical receiver 350 to theoptimal threshold for the received power level in order to maximize thesignal-to-noise ratio of the optical receiver 350.

The optical receiver 350 includes an electronic dispersion compensationcircuit 376 having an input 378 that is electrically connected to anoutput 380 of the decision threshold circuit 366. The electronicdispersion compensation circuit 376 compensates for the effects ofdispersion by reconstructing the dispersed optical signals. Dispersioncan severely degrade signals in the optical fiber transmission systems100, 150 that are described in connection with FIG. 1 and FIG. 2.Several different types of dispersion can occur in these optical fibertransmission systems.

For example, chromatic dispersion can occur in WDM optical fibertransmission systems. Chromatic dispersion is caused by differences inthe speed at which signals having different wavelengths travel in theoptical fiber link. Chromatic dispersion generally decreases theacceptable transmission distance as the square of the bit rate.

Polarization mode dispersion (PMD) occurs when the orthogonalpolarization components of the optical signal travel at different ratesin the optical fiber link. Polarization mode dispersion results fromasymmetries in the optical fiber core. Polarization mode dispersioncauses a statistical disruption in network operation and, consequently,limits the transmission distance.

Signal degradation caused by these dispersions, if uncompensated, cancorrupt the received optical signal by broadening the pulses in theoptical signal, which causes Inter Symbol Interference (ISI). The ISIwill eventually degrade the signal quality enough for the signal to fallbelow the acceptable threshold for service. Thus, these dispersions canlimit the possible bandwidth-distance product in the optical fiber linksand can cause service interruptions.

The dispersion compensation circuit 376 includes at least one activefilter. There are many different types of active filters know in the artthat are suitable for electronic dispersion compensation. For example,the active filter can be a Finite Impulse Response (FIR) filter, such asa Feed Forward Equalizer (FFE) filter. Such filters sample the receivedsignal, after electro-optic conversion by the photo-detection circuit352. Different delayed samples are scaled and then summed once persample clock. The length of the FIR filter (i.e. the number of taps) isrelated to the amount of ISI that is incurred during transmission.

The dispersion compensation circuit 376 can also include a DecisionFeedback Equalizer (DFE) filter that is used with the FFE filter tofurther reduce the ISI in the optical signal. The DFE filter takes thedecisions from the FFE filter as its input. The output of the DFE filteris combined with the output of the FFE filter and is fed back to theinput of the DFE filter. The clock and data are then recovered from thedispersion compensated signal.

The optical receiver 350 also includes a demodulator 382 having an input384 that is electrically connected to an output 386 of the dispersioncompensation circuit 376. The demodulator 382 demodulates thereconstructed optical signal and recovers the transmitted data. Thedemodulator 382 generates the recovered data at an output 388.

Equivalents

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A multi-mode optical fiber link comprising: a first spatial modeconverter having an input that is coupled to an output of a single modeoptical fiber, the first spatial mode converter conditioning a modalprofile of an optical signal propagating from the single mode opticalfiber to the first spatial mode converter; a multi-mode optical fiberhaving an input that is coupled to an output of the first spatial modeconverter; and a second spatial mode converter having an input that iscoupled to an output of the multi-mode optical fiber and an output thatis coupled to a second single mode optical fiber, the second spatialmode converter reducing a number of optical modes in the optical signal,wherein both the first and the second spatial mode converters increasean effective modal bandwidth of the optical signal.
 2. The optical fiberlink of claim 1 further comprising an optical source having an outputthat is coupled to an input of the single mode optical fiber, theoptical source generating the optical signal having a wavelength.
 3. Theoptical fiber link of claim 2 wherein the optical source comprises anintensity modulated optical source.
 4. The optical fiber link of claim 2wherein the optical source comprises an electro-absorption modulatedlaser.
 5. The optical fiber link of claim 2 wherein the optical sourcecomprises an integrated laser modulator.
 6. The optical fiber link ofclaim 2 further comprising a second optical source having an output thatis coupled to the input of the single mode optical fiber, the secondoptical source generating a second optical signal having a secondwavelength at the output.
 7. The optical fiber link of claim 2 furthercomprising a second optical source having an output that is coupled tothe output of the second spatial mode converter, the second opticalsource generating a second optical signal having a second wavelength atthe output.
 8. The optical fiber link of claim 1 wherein at least one ofthe first and the second spatial mode converters comprise a fusionsplice between the multi-mode optical fiber and a respective one of thesingle mode optical fiber and the second single mode optical fiber. 9.The optical fiber link of claim 1 wherein at least one of the first andthe second spatial mode converters comprises a lens imaging systemhaving refractive and diffractive elements.
 10. The optical fiber linkof claim 1 wherein the second spatial mode converter reduces a number ofhigher-order modes propagating in the optical signal.
 11. The opticalfiber link of claim 1 wherein the second spatial mode converter reducesa number of lower-order modes propagating in the optical signal.
 12. Theoptical fiber link of claim 1 wherein the second spatial mode converterreduces both a number of lower-order and a number of higher-order modespropagating in the optical signal.
 13. The optical fiber link of claim 1wherein the first spatial mode converter optically couples the singlemode optical fiber to the multi-mode optical fiber so as to achieve apredetermined offset between a core of the single mode optical fiber anda core of the multi-mode optical fiber.
 14. A method of increasingeffective modal bandwidth of an optical signal transmitting through amulti-mode optical fiber, the method comprising: spatial mode convertingan optical signal, thereby reducing modal dispersion and increasing aneffective bandwidth of the optical signal; propagating the spatiallymode converted optical signal through a multi-mode optical fiber; andspatial mode converting the spatially mode converted optical signalpropagated through the multi-mode optical fiber, thereby furtherreducing modal dispersion and further increasing the effective bandwidthof the optical signal.
 15. The method of claim 14 further comprisinggenerating the optical signal.
 16. The method of claims 15 wherein theoptical signal is generated with relatively low time varying phase andsideband information.
 17. The method of claim 14 wherein the spatialmode converting at least one of the optical signal and the spatiallymode converted optical signal reduces changes in effective modalbandwidth of the optical signal that are caused by thermal variations inthe multi-mode optical fiber.
 18. The method of claim 14 wherein thespatial mode converting at least one of the optical signal and thespatially mode converted optical signal reduces changes in effectivemodal bandwidth of the optical signal that are caused by polarizationeffects in the multi-mode optical fiber.
 19. The method of claim 14wherein the spatial mode converting at least one of the optical signaland the spatially mode converted optical signal reduces changes ineffective modal bandwidth of the optical signal that are caused bymechanical stress in the multi-mode optical fiber.
 20. The method ofclaim 14 wherein the spatial mode converting at least one of the opticalsignal and the spatially mode converted optical signal reduces changesin effective modal bandwidth of the optical signal that are caused byoptical fiber splices in the multi-mode optical fiber.
 21. The method ofclaim 14 wherein the optical signal comprises more than one opticalwavelength.
 22. A multi-mode optical communication system comprising: anoptical transmitter that generates an optical signal at an output; afirst spatial mode converter having an input that is coupled to theoutput of the optical transmitter, the first spatial mode converterconditioning a modal profile of the optical signal; a multi-mode opticalfiber having an input that is coupled to an output of the first spatialmode converter; a second spatial mode converter having an input that iscoupled to an output of the multi-mode optical fiber, the second spatialmode converter reducing a number of optical modes in the optical signal,wherein both the first and the second spatial mode converters increasean effective modal bandwidth of the optical signal; and an opticalreceiver having an input that is coupled to an output of the secondspatial mode converter, the optical receiver receiving the opticalsignal.
 23. The communication system of claim 22 wherein the opticaltransmitter comprises an electro-absorption modulated laser.
 24. Thecommunication system of claim 22 wherein the optical transmittercomprises an integrated laser modulator.
 25. The communication system ofclaim 22 wherein the optical signal comprises more than one opticalwavelength.
 26. The communication system of claim 22 wherein the secondspatial mode converter reduces a number of higher-order optical modes inthe optical signal.
 27. The communication system of claim 22 wherein thesecond spatial mode converter reduces a number of lower-order opticalmodes in the optical signal.
 28. The communication system of claim 22wherein the multi-mode optical fiber comprises at least one section ofsingle mode optical fiber.
 29. The communication system of claim 22wherein the optical receiver comprises an active filter thatreconstructs dispersed optical signals received by the optical receiver.30. The communication system of claim 22 wherein the optical receiverautomatically adjusts at least one receiver parameter in order tocompensate for changes in an average power of the received opticalsignal.
 31. The communication system of claim 30 wherein the opticalreceiver automatically adjusts the at least one receiver parameter so asto maintain a substantially constant bit error rate as the average powerof the received optical signal changes.
 32. The communication system ofclaim 30 wherein the at least one receiver parameter comprises receiversensitivity.
 33. The communication system of claim 22 wherein theoptical transmitter comprises an optical intensity modulator, at leastone parameter of the optical intensity modulator is chosen to suppressat least one of phase and sideband information in the optical signal.34. The communication system of claim 33 wherein the at least oneparameter of the optical intensity modulator comprises a bandwidth ofthe optical intensity modulator.
 35. The communication system of claim33 wherein the at least one parameter of the optical intensity modulatorcomprises an absorption spectrum of the optical intensity modulator. 36.The communication system of claim 33 wherein the at least one parameterof the optical intensity modulator comprises an extinction ratio of theoptical intensity modulator.
 37. The communication system of claim 33wherein the at least one parameter of the optical intensity modulatorcomprises an absorption coefficient of the optical intensity modulator.38. The communication system of claim 33 further comprising an opticalisolator that substantially eliminates reflected optical signals frompropagating into an output of the optical intensity modulator.
 39. Amulti-mode optical communication system comprising: means for spatialmode converting an optical signal, thereby reducing modal dispersion andincreasing an effective bandwidth of the optical signal; means forpropagating the spatially mode converted optical signal through amulti-mode optical fiber; and means for spatial mode converting thespatially mode converted optical signal propagated through themulti-mode optical fiber, thereby further reducing modal dispersion andfurther increasing the effective bandwidth of the optical signal.